1. Field of the Invention
The present invention relates to an autofocusing apparatus suitable for a sighting telescope incorporated in a surveying instrument such as an automatic level, a transit, a theodolite, etc.
2. Description of the Related Art
A conventional surveying instrument such as an automatic level (auto-level), a transit, a theodolite, etc., is generally provided with a sighting telescope, a level and a measuring device which measures a rotational angle, a descending angle, and an ascending angle, etc. When this type of surveying instrument is used, the sighting telescope is positioned horizontally and subsequently the horizontal and vertical adjustments for the sighting telescope are performed. Thereafter, the sighting telescope is aimed at a reference object or a reference point so that the reference object is sighted by a surveyor.
For instance, the optical system of the sighting telescope of an automatic level includes an objective lens group, a focusing lens group and an eyepiece, arranged in that order from the object side. The position of the focusing lens group is adjusted depending on the object distance, so as to form a sharp object image on the reticle provided on the focal plane. The object image formed on the reticle can be viewed through the eyepiece.
In a conventional sighting telescope provided with an autofocusing system, immediately after the AF start button of the autofocusing system is depressed by the user, the focusing lens group of the system is driven from a current position (e.g., initial position) to another position to bring a sighting object into focus. According to a phase-difference detection AF system, a focal point which is firstly detected by the AF system is regarded as an actual focal point for the sighting object, so that the AF system drives the focusing lens group to an axial position thereof which corresponds to the initially detected focal point to stop the focusing lens group thereat.
According to such an autofocusing control performed by the autofocusing system of a conventional sighting telescope, when auto-focusing the telescope on a prism such as a corner-cube reflector, either the frame which supports the prism or a telescope""s image reflected off the prism is brought into focus. In other words, it is unreliably determined as to which of the two images (an image of the frame or an image of the telescope) is brought into focus by the autofocusing system. Specifically when the telescope is seen from the front, the front face of the objective lens of the telescope tends to be seen as a dark image while the contrast of the frame of the telescope is generally high, so that the telescope is often auto-focused mistakenly on an image of itself reflected off the prism, rather than the corner-cube reflector or corner-cube prism.
The primary object of the present invention is to provide an autofocusing apparatus of a sighting telescope which makes it possible to reliably focus the telescope on a sighting object, and in particular, a prism such as a corner-cube reflector.
To achieve the object mentioned above, according to an aspect of the present invention, there is provided an autofocusing apparatus of a telescope for automatically focusing the telescope on an object viewed within a field of view of the telescope through a distance-measuring zone arranged in the field of view, the autofocusing apparatus including: a multi-point distance measuring device which divides the distance-measuring zone into at least three distance-measuring zones to detect an object distance on each of the distance-measuring zones; a lens driver which drives a focusing lens group of the telescope along an optical axis thereof; and a controller which controls the lens driver so that the focusing lens group is moved to perform an autofocusing operation in accordance with a result of the each object distance detected by the multi-point distance measuring zones. The controller compares the each object distance detected by the multi-point distance measuring device with each other. If the controller determines that two object distances respectively detected on two distance-measuring zones among the aforementioned at least three distance-measuring zones are substantially the same, the two distance-measuring zones being separate from each other by at least one distance-measuring zone positioned therebetween, and that an object distance detected on one of the at least one distance-measuring zone is substantially double the distance of each of the two object distances which are substantially the same, the controller determines that each of the two object distances is a valid object distance so that the autofocusing operation is performed in accordance with the valid object distance.
Preferably, if there are no object distances detected on the aforementioned at least one distance-measuring zone which are substantially double the distance of each of the two object distances, the controller determines the shortest object distance among all of the object distances detected by the multi-point distance measuring device as a valid object distance so that the autofocusing operation is performed in accordance with the shortest object distance.
Preferably, the autofocusing apparatus is incorporated in a surveying instrument.
Preferably, there is further provided a memory in which the object distances measured by the multi-point distance measuring device are temporarily stored.
Preferably, the multi-point distance measuring device includes a phase-difference detection AF sensor.
Preferably, the phase-difference detection AF sensor includes a pair of line sensors.
Preferably, any two adjacent distance-measuring zones among the at least three distance-measuring zones overlap each other by a predetermined amount.
Preferably, the distance-measuring zone extends horizontally along the field of view of the telescope.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 10-291955 (filed on Oct. 14, 1998) which is expressly incorporated herein by reference in its entirety.